I really liked the LED explanation at the 4:00 mark. Can anyone who is familiar with semiconductor physics opine on how well this explanation models the reality?
When he talks about the electrons "feeling" the neighboring atoms, he's talking specifically about a result that follows from the materials being crystalline, that is, having regular ordered structure. The regular structure gives rise to a periodic potential. You plug that periodic potential into the Schrodinger equation and apply continuity conditions and translational symmetry to the wavefunction. Computing the solutions to the Schrodinger equation with those conditions reveals that there are allowed and disallowed energy levels, and also reveals the relationship between energy and momentum in the crystal lattice. You can step through this by reading the wikipedia page on the Kronig-Penney Model. This depends on the periodicity, which obviously can change depending on direction in a crystal.
His explanation, and the result that "the" band gap is a single number, isn't dishonest because when we grow semiconductor devices, we grow them such that the crystal is oriented such that current flows in the desired direction, so that simple result holds true.
Even his portrayal of the bands leaning down as potential/voltage is applied mirrors how potential change is shown in diagrams of semiconductor devices, see Streetman and Banerjee - Solid State Electronic Devices.
Trying to make a long story short, it can be the case that in order to transition to another energy level, an electron also has to exchange momentum with something, usually the lattice in the form of quantized vibrations. Photons carry energy but almost no momentum, so an indirect semiconductor (one that requires both energy and momentum exchange for a transition to the conduction band) is usually an abysmal choice for optoelectronics.
The existence of infrared LEDs seems to indicate to me that infrared light can exist without heat.
The existence of infrared thermometers seems to imply that hot stuff radiates infrared light, at least usually.
So my question is, is there any case where heat does not cause infrared radiation? What are those cases? Some special materials? Special colours(perhaps outside the visible spectrum)?
However, what the exact mixture is depends on the temperature of the body. As the body gets hotter, the 'peak' wavelength, i.e. the wavelength whose "amount" is highest in the mixture decreases.
Objects at room temperature emit most of their energy outside of the visible spectrum, so they are not 'visible' in the dark. However, as you heat them up, the radiation mixture moves towards lower wavelengths, closer to infra-red. Heat it up further and things become red hot, yellow, blue hot and so on.
Infra-red LEDs produce light of the the specific infrared wavelength through semi-conductors. They have nothing to do with the black-body radiation one associates with 'hot' objects.
https://en.wikipedia.org/wiki/Emissivity
> Real objects never behave as full-ideal black bodies, and instead the emitted radiation at a given frequency is a fraction of what the ideal emission would be. The emissivity of a material specifies how well a real body radiates energy as compared with a black body. This emissivity depends on factors such as temperature, emission angle, and wavelength. However, it is typical in engineering to assume that a surface's spectral emissivity and absorptivity do not depend on wavelength so that the emissivity is a constant. This is known as the gray body assumption.
https://en.wikipedia.org/wiki/Black-body_radiation
In a previous life I worked for a lab designing a passive radiator that was reflective to visible light but absorptive/emissive in the atmospheric window (where the atmosphere is completely transparent). This material had the fun effect of absorbing energy via conduction/convection and then sending it out into space, maintaining a temperature slightly below ambient.
While a great introduction to semiconductor behavior this does gloss over a very important detail namely direct vs indirect semicondoctors as some others have mentioned. In the video the detail that's glossed over relates to the nature of crystals, namely that they're highly ordered repeating structures but that they don't look the same when viewed from every direction. This means that there isn't a single band-gap but multiple ones depending on the direction of the crystal you're contemplating.
At this point you may reasonably ask why the direction matters and now we unfortunately get deep into the weeds with quantum mechanics again. When a single photon is absorbed in the semiconductor system both momentum and energy must be conserved. The momentum of the photon for something like the Silicon bandgap is quite small (something like the equivalent of an electron traveling at 1500m/s) while the momentum of room-temperature conduction electrons is substantially faster [2] so as a very slight simplification transitions due to the absorption of photons are not accompanied by a change in momentum and so we only care about the band structure (and the accompanying free carriers) associated with a particular crystal direction.
In particular in Silicon you have what's called an indirect bandgap, namely the minimum energy conduction band electrons have a different momentum from the valence band holes ([3]) and as a consequence while you can _absorb_ a photon in order to make a detector you cannot make it efficiently _emit_ a photon as an LED should (something the video got wrong).
None of this matters for the heart of the video, which focuses blue LEDs in the GaN materials system which is definitely a direct bandgap material, however if someone does manage to create a manufacturable light emitter in pure Silicon expect an absolute revolution with regards to optical computing and photonics. (Not for lack of trying, this has been the holy grail for at least 20 years, possibly longer)
[1] https://en.wikipedia.org/wiki/Quasiparticle [2] https://www.chu.berkeley.edu/wp-content/uploads/2020/01/Chen... [3] https://www.iue.tuwien.ac.at/phd/wessner/node31.html
A cool chance to show the importance of determination!
I mean, I could explain how they worked in the same ways that they were explained to me, but I couldn't connect those explanations to a true physical understanding.
But thanks to this, I finally actually understand.
Also, the LED story was fascinating.